The present invention generally relates to the field of optical measurement and/or inspection techniques and more specifically relates to an auto-focus method and device, particularly useful when viewing non planar surfaces.
Auto focusing is an essential feature in many automated inspection fields such as the computer chip industry, biomedical research, data reading/recording in optical information carriers, etc. Specifically, when analyzing samples in multiwell plates including a plurality of wells on a single plate, auto focusing of a microscope viewing the contents of the wells can enable more efficient work procedures as the operator need not focus the objective on each well in the plate separately.
Various auto focusing methods for inspection of a multiwell plate have been disclosed in the past, such as in U.S. Pat. No. 7,109,459. However, when using wells having a non-planar bottom, such as a multiwell plate having wells with a U-shaped bottom, such as are used, for example, for growing living cells into spheroids, existing autofocus methods may require image analysis which is time-consuming.
In some cases, a multiwell plate is analyzed or is visualized using an oil immersion lens having a high numerical aperture. In such cases, focusing the objective using laser based autofocusing methods as described in U.S. Pat. No. 7,109,459 is difficult or impossible, as the similar refraction index of the lens and solution in the wells cancels one of the refraction points and a corresponding scanning peak, causing difficulty in focusing the objective lens.
Thus there are needs: (a) for auto focusing a microscope on a multiwell plate, which is suitable for multi-well plates having wells with a non-planar bottom surface; (b) to separate focusing process from scanning process to allow higher scanning and imaging speed; and (c) for a method for auto focusing a microscope using an oil immersion lens on a multiwell plate.